Oxoacids are basically the acids that contain oxygen. Phosphorus forms a number of oxoacids, for example: H3PO4, H3PO3, etc. In oxoacids of phosphorus, phosphorus is tetrahedrally surrounded by other atoms. Generally, all these acids are known to form at least one P=O bond and one P–OH bond. P–P or P–H bonds are also found in addition to P=O bonds and P–OH bonds in oxoacids of phosphorus where the oxidation state of phosphorus is less than +5. These acids are generally seen to disproportionate to higher and lower oxidation states. For example, phosphorous acid on heating disproportionates to give phosphoric acid and phosphine.\(4H_3PO_3~\rightarrow~ 3H_3PO_4~ +~ PH_3\)
The P-H bonds in oxoacids are not ionisable to give H+ ions whereas the H atoms which are attached with oxygen in P-OH form are ionisable. Hence we can say that only the H atoms attached with oxygen cause basicity. As a result phosphorous acid, H3PO3 is dibasic due to the presence of two P-OH bonds whereas phosphoric acid, H3PO4 is tribasic due to the presence of three P-OH bonds. Oxoacids of phosphorus having P-H bonds have strong reducing properties. For example: hypophosphorous acid containing two P-H bonds acts as a good reducing agent.\(4 ~AgNO_3~ +~ 2H_2O~ +~ H_3PO_2~\rightarrow~ 4Ag~ + ~4HNO_3~ + ~H_3PO_4\)
Few popular oxoacids of phosphorus
Phosphorus acid, H3PO3: Phosphorous acid is a diprotic acid that is, it ionizes two protons. It is better described with the structural formula HPO(OH)2. Phosphorous acid is prepared by hydrolysis of phosphorus trichloride with acid or steam.\(PCl_3 ~+~ 3 ~H_2O~ \rightarrow~ HPO(OH)_2~ +~ 3~HCl\)
Phosphoric acid, H3PO4: Phosphoric acid is a triprotic acid that is, it ionizes three protons. It is a non-toxic acid, when pure and is a solid at room temperature and pressure. Phosphoric acid is prepared by adding sulfuric acid to tricalcium phosphate rock:\(Ca_5(PO_4)_3X~ +~ 5 ~H_2SO_4~+~ 10~ H_2O~ →~ 3 H_3PO_4~ +~ 5 ~CaSO_4.2H_2O ~+~ HX\)
X can be F, Cl, Br and OH
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